1. Field of the Invention
The present invention concerns a transmission relay system, or re-transmitter unit, which receives a first signal on a first carrier frequency from a remote point to re-transmit on a second carrier frequency a second signal in the form of a beam covering a geographical area. The first and second carrier frequencies can be the same and the first and second signals modulating these two carrier frequencies can be the same signal.
The invention finds applications in satellite telecommunication systems, for example. Embodiments of the invention apply to a surveillance satellite system or a low altitude satellite system. The signal carried by the first carrier frequency is transmitted by a ground station to a satellite. A transmission relay system in the latter re-transmits a signal carried by the second carrier frequency.
2. Description of the Prior Art
One prior art solution uses beam forming networks (BFN) which adjust the phase of the signal produced by each radiating element to form a beam that is transmitted with a particular wavefront defining the orientation of the beam.
As shown in FIG. 1, in some satellite telecommunication networks at least one satellite 1 re-transmits each signal UP received from a base station 2 in the opposite direction to this received signal. Thus the satellite 1 is used as a relay to form a beam DOWN covering a geographical area centered on the base station from which it has received a signal. In such applications conventional beam forming networks can be used. This then implies complex processing on board the satellite for the application concerned.
A first objective of the invention is to remedy this drawback by providing a transmission relay system of low technical complexity for an application of the type limited to re-transmitting the received signal in the opposite direction.
A second objective of the invention is to provide a particularly advantageous transmit and receive antenna geometry.